The invention relates to a measuring apparatus for contactlessly determining the position of two components which are displaceable relative to each other with a translatory movement, in particular for determining the position of a displacement piston which is fitted in a cylinder and which is drivingly connected to clamping jaws of a power-operated chuck, wherein the one component is in the form of a measurement value sender and the other component is in the form of a sensor co-operating therewith, the sensor comprises a plurality of measuring heads which are arranged over the displacement range of the measurement value sender and which are connected to a computing unit which has an evaluation circuit and in which there is stored in a memory the signal configuration of the individual measuring heads, which is ascertained upon calibration of the measuring apparatus in dependence on the position of the measurement value sender, and the position of the displaceable component can be determined after an axial displacement movement of the measurement value sender by comparison of the signals respectively produced thereby in the measurement field with the stored signal configuration.
An electronic position detector of that kind is known from DE 32 44 891 A1. In that arrangement the sensor comprises a plurality of coils which are disposed on different ferrite members as carriers in the same mechanical arrangement with a defined spacing and which co-operate with a measurement value sender which is also in the form of a coil. If a voltage pulse or an ac voltage is applied to the individual coils in succession or at the same time, the signals in the individual coils differ, depending on the position in which the measurement value sender is disposed, as a position indicator, relative to one of the oppositely disposed coils. If the individual coils are interrogated for example by means of an electronic multiplexer, the different signal inputs are directly a measurement in respect of the position of the measurement value sender.
Feed lines to the displaceable measurement value sender are admittedly avoided, but in many cases measurement inaccuracies have to be tolerated as the magnetic fields of the coils forming the sensor overlap and thus in many situations a precise association of the measurement value sender is not possible.
Accordingly, the object of the invention is to provide a measuring apparatus for contactlessly determining the position of two components which are displaceable relative to each other with a translatory movement, which not only can be produced in an extremely economical fashion but with which incorrect measurements are reliably also avoided. The invention seeks to provide that in this case a magnetic shock as well as external influences which can result in measurement inaccuracies do not have to be tolerated; on the contrary the invention seeks to provide that the measuring apparatus is operational with an associated consumer so that precise determination of the respective axial position of the component to be monitored is made possible in a very simple fashion.
In the measuring apparatus designed in accordance with the invention for contactiessly determining the position of two components which are displaceable relative to each other with a translatory movement, that is attained in that the measurement value sender is in the form of a switching ring and comprises a metallic, preferably ferromagnetic material, and that the switching ring is provided on the end face towards the sensor with a profiling formed by at least one recess machined therein.
In this case, the flanks of the recess can be perpendicular to the axis or of a trapezoidal shape extending inclinedly outwardly or inwardly relative to the face of the switching ring or can be of a convexly or concavely curved configuration, in which respect it is appropriate for two recesses which are arranged at a lateral spacing from each other to be formed in the face of the switching ring.
In the case of a switching ring with recesses which are incorporated in the face thereof perpendicularly to the axis, in relation to the face thereof, it is desirable for the recesses and the projections formed thereby to be of the same width.
The lateral spacing of the measuring heads can be different in relation to the lateral spacing of two flanks of two projections, which are directed in the same direction. In accordance with a preferred configuration the spacing of the measuring heads in relation to the spacing of the flanks should be in a ratio of three to four.
The measuring heads should be arranged equidistantly over the displacement range of the switching ring, preferably in a line. The measuring heads which produce an electromagnetic measuring field composed of individual fields can each also be formed from a ferrite shell core and a coil pack.
If a measuring apparatus is designed in accordance with the invention, it is possible to ascertain the respective position of the component which is displaceable with a translatory movement, without involving a high level of structural complication and expenditure. More specifically, along the measuring distance, an electromagnetic measuring field composed of the respective individual fields is produced by the inductive measuring heads which can comprise ferrite shell cores with for example on average 3.3 mm for inductive proximity switches and a coil pack on copper enamel wire of 0.028 mm with twice 170 windings. When the switching ring is in the working region of the measuring heads, they are influenced by way of the action of the switching ring on the associated electromagnetic individual fields. When dealing with ferromagnetic or electrically conductive object materials, that results in a variation in the inductance of the measuring head and also eddy current losses, in which respect basically the two effects act in opposition. When dealing with non-ferromagnetic object materials in contrast, only eddy current losses occur. Those effects can be measured with circuits which are suitable for that purpose. Taking the actual attenuation profile of the individual measuring heads, using an evaluation algorithm, it is possible to calculate the current axial position of the switching ring, more specifically independently of the angle of rotation and within certain limits of the radial spacing between the switching ring and the measuring head.
The face of the switching ring which is manufactured from a commercially available steel is desirably to be profiled in such a way that the surface thereof forms three projections. They produce an increase in the level of measuring accuracy as more signal edges are produced for evaluation, per unit of length. That increase is used in order to minimise the number of measuring heads and to be able to make the spacing thereof relative to each other as large as possible. In that way on the one hand the measuring system becomes less expensive, while on the other hand this avoids mutual influencing the measuring heads.
In practice however it is not possible to achieve an ideal triangular signal configuration as inter alia the maxima and the minima markedly flatten off. Signals with a horizontal or a vertical configuration are unsuitable for evaluation as they do not contain clear travel information. There are however always two measuring signals between y=0.25 and y=0.75, accordingly in a region in which a linear rise can in practice also be produced. With the respectively largest individual signal y greater than 0.75 it is thus possible to calculate a plurality of possible coarse positions and with the second and third largest signal (0.25 less than y less than 0.75), by virtue of linear interpolation and signal linking, it is possible to calculate the definitive clear position, having regard to the coarse position.
The signals produced by the individual measuring heads can be processed by a standardisation procedure in such a way that common-mode disturbances which act on all measuring heads are almost completely eliminated. Such disturbances include inter alia fluctuations in temperature, changes in the properties of the steel of the switching ring under the influence of temperature or centrifugal force, ageing and temperature drift in the analog electronics and a variation in the spacing from the switching ring to the measuring heads.
Thus, a comparison of the pre-processed signal configurations with the signal configurations which are stored in a memory and which were ascertained upon calibration of the measuring apparatus makes it easily possible to determine the respective position of the component which is displaceable with a translatory movement, namely the switching ring, and accordingly, in dependence thereon, also the position of the clamping piston whose position changes upon a loss in clamping force. In that respect it is immaterial whether the component which is displaceable with a translatory or linear movement performs only such a movement or in that respect also rotates. A rotational movement has no influence on the signal configuration. The clamping force of a power-operated chuck is accordingly to be easily monitored by way of a change in position of the displacement piston which acts on the clamping jaws thereof.